The present invention relates to a method and apparatus for echo estimation and suppression in telephony systems.
Echo is a problem related to the perceived speech quality in telephony systems with long delays, e.g. telephony over long distances or telephony systems using long processing delays, like digital cellular systems. The echo arises in the four-to-two wire conversion in the PSTN/subscriber interface (PSTN=Public Switched-Telephone Network). To remove this echo, echo cancellers are usually provided in transit exchanges for long distance traffic, and in mobile services switching centers for cellular applications.
Due to the location of the echo canceller it is made adaptive; the same echo canceller is used for many different subscribers in the PSTN. This adaptation is necessary not only between different calls, but also during each call, due to the non-fixed nature of the transmission network, e.g. phase slips, three-party calls, etc.
The main part of an echo canceller is an adaptive filter. The filter generates a replica of the echo, which is subtracted from the near end signal. Due to imperfect knowledge of the echo generating system, the estimated echo signal always contains errors. Hence, in practice, the echo attenuation obtained by using an adaptive filter is usually at most approximately 30 dB. For long time delays this attenuation is not enough, and in order to minimize the audible effects of these errors, a residual echo suppressor is used. The purpose of the echo suppressor is to further suppress the residual signal whenever this signal is dominated by the errors in the echo estimate. This is done by blocking the output of the echo canceller for certain levels of the output signal.
Reference [1] describes an echo canceller provided with an echo suppressor in the form of an adaptive center clipper. The echo estimate produced by the echo canceller is used to control, via signal processing means, the threshold, and thereby the clipping window, of this adaptive clipper. If the power of the residual signal falls below the adaptive threshold, the residual signal is blocked or clipped, otherwise the residual signal is passed through the adaptive clipper without modification. However, the residual signal contains not only residual echo, but also background noise produced at the near end subscriber. Occasionally residual echo samples and background noise samples add constructively, and the resulting residual signal may therefore exceed the threshold. The result is undesirable sporadic transmissions of residual signals containing residual echo, which can be very annoying.
Echo cancellers in transit exchanges or mobile switching centers are shared devices, which can be used towards any of a multitude of subscriber lines or echo paths. The characteristics of these echo paths can vary substantially. Particularly, the delay, the power level and the linearity of the echo may vary. This results in great difficulty in determining proper, fixed, values for the echo suppressor or non-linear processor (NLP). Hence, it is desirable to dynamically estimate the amount of echo power forwarded to the echo suppressor.
Another possible cause for echo is the acoustical and mechanical coupling of the loudspeaker and the microphone in telephone handset. For a mixed analog and digital network, such as a PSTN with analog two wire subscriber lines, this echo usually causes no audible disturbance. This is so because the echo from the handset is masked by the usually much stronger electrical echo in the four-to-two wire conversion. Furthermore, by applying an echo canceller for the latter echo, also the echo from the handset will be controlled.
On the other hand, in digital telephony systems even a low level of acoustical and mechanical cross-talk might become audible if the transmission delay is large. This has been recognized in the specification of several digital systems. As an example, the GSM specification (GSM=Global System for Mobile Communications) requires an echo loss of at least 46 dB between loudspeaker and microphone, as measured in the Switching System. This echo loss should be measured with the handset earpiece sealed to the ear, and with the volume control of the telephone set to its maximum. However, in normal use there is usually a gap between ear and earpiece, which might result in a lower echo loss. Thus, although a handset may fulfill the GSM specification, in practice there is still a possibility that echo might be observed during normal use. Hence, it may be desirable to detect and cancel also this echo from the switching system.
An object of the present invention is to provide an echo power estimation and suppression method and apparatus that avoids the above mentioned problems.
Briefly, the present invention is based on the finding that there is a correlation between the power of a delayed echo and the power of the signal generating the echo. This correlation is used to build a model capable of estimating echo return loss and delay. These parameters are used to form a clip level used as a dynamic threshold for controlling an echo suppressor. The method is also computationally efficient.